The invention relates generally to time-division switching systems and more particularly to a telephone system for switching PCM (Pulse Code Modulated) data words among large numbers of time-division multiplex lines.
It is the function of a telephone switching system to establish communication connections between calling lines or trunks and called lines or trunks. Systems are known in the prior art in which analog signals from a plurality of lines or trunks are converted to PCM data words and are multiplexed onto a single transmission line having recurring time frames each comprising a plurality of channels. A channel is an identifiable time period on the time separated transmission line which occurs once in each time frame of the line. Known prior art systems typically have 24 channels per time frame and data words from 24 independent lines or trunks are transmitted during each time frame. PCM data words may be switched among multiplex lines by selectively transferring PCM data words from the various channels of an input multiplex line to a predefined channel on one of a plurality of output multiplex lines. The transfer of data words from input multiplex lines to output multiplex lines may be accomplished by means of a space-division network which is reconfigured at a predetermined rate compatible with the rate at which the data is received from input multiplex lines.
A space-division switching network which is utilized to transfer information among a large number of input and output multiplex lines must have a high capacity and low blocking. Such a switching network is typically physically large which results in time delays of information transmitted therethrough and these delays are large relative to the time duration of an individual channel. When the sum of the transmission time delays and the time period used to transmit PCM data words becomes greater than the duration of a channel, a PCM data word is not able to traverse the entire switching network during the time of one channel. One solution suggested by the prior art is to divide the network into stages each having a relatively small time delay with respect to the time period of a channel and to provide a data buffer memory between each of these stages. Data is then transmitted through one stage of the network and stored in a data buffer memory within the time period of a channel. The data is, in this manner, transmitted and buffered through each successive network stage with each transmission and buffering step requiring the time period of a channel. When operating in this manner, each network stage functions in channel synchronism. This solution requires the use of a large number of buffer memories as well as a large amount of control circuitry to control the system.